Patent application title: ABSOLUTE DISTANCE METER WITH OPTICAL SWITCH

Abstract:

An absolute distance meter (ADM) that determines a distance to a target
includes a light source that emits an emitted light beam. The ADM also
includes a fiber switching network having at least one optical switch
that switches between at least two positions in response to a switch
control signal, a first one of the positions enabling a measure mode in
which the emitted light beam is emitted from the fiber switching network
towards the target and is reflected back as a measure light beam into the
fiber switching network, a second one of the positions enabling a
reference mode in which the light beam comprises a reference light beam
within the fiber switching network. The ADM further includes a single
channel detector that detects the measure and reference light beams in a
temporally spaced multiplexed manner and provides an electrical signal
which corresponds to the detected measure and reflected light beams.
Also, the ADM includes a single channel signal processor that processes
the electrical signal and provides a conditioned electrical signal in
response thereto, and a data processor that processes the conditioned
electrical signal to determine the distance to the target.

Claims:

1. An absolute distance meter that determines a distance to a target,
comprising:a light source that emits a light beam;a fiber switching
network having at least one optical switch that switches between at least
two routes in response to a switch control signal, a first one of the
routes enabling a measure mode in which the light beam is emitted from
the fiber switching network through an end of an optical fiber towards
the target and is reflected back through the end of the optical fiber as
a measure light beam and back into the fiber switching network, a second
one of the routes enabling a reference mode in which the light beam
comprises a reference light beam within the fiber switching network;a
single channel detector that detects the measure and reference light
beams in a temporally spaced multiplexed manner and provides an
electrical signal which corresponds to the detected measure and reference
light beams;a single channel signal processor that processes the
electrical signal and provides a conditioned electrical signal in
response thereto;electronics that controls timing of the switch control
signal; and a data processor that processes the conditioned electrical
signal to determine the distance to the target.

2. The absolute distance meter of claim 1, wherein the light source
comprises a laser or superluminescent diode and wherein the light beam is
a laser light beam or a superluminescent diode light beam.

3. The absolute distance meter of claim 1, wherein the single channel
signal processor provides a modulation signal to the light source to
modulate the light beam power, polarization, wavelength, phase, or
combinations thereof in a manner that is sinusoidal, pulsed, chirped, or
combinations thereof.

4. The absolute distance meter of claim 1, wherein the single channel
signal processor provides the switch control signal to control the
switching of the at least one optical switch between the measure mode
route and the reference mode route.

5. The absolute distance meter of claim 1, wherein the absolute distance
meter is for use within a laser tracker, total station, laser scanner, or
handheld device.

6. The absolute distance meter of claim 1, wherein the fiber switching
network further comprises:at least one fiber optic coupler through which
the emitted light beam and the measure and reference light beams pass;
anda partial fiber retroreflector;wherein the at least one fiber optic
coupler is optically connected to the single channel detector, to the
light source, and to the at least one optical switch;wherein in the
measure mode the emitted light beam is sent from the light source through
the at least one fiber optic coupler to the at least one optical switch
that is in the measure mode route and to the target, and wherein the
measure light beam from the target passes through the at least one
optical switch that is in the measure mode route and through the at least
one fiber optic coupler and to the single channel detector; andwherein in
the reference mode the emitted light beam is sent from the light source
through the at least one fiber optic coupler to the at least one optical
switch that is in the reference mode route and to the partial fiber
retroreflector, and wherein the reference light beam reflected from the
partial fiber retroreflector passes through the at least one optical
switch that is in the reference mode route and through the at least one
fiber optic coupler and to the single channel detector.

7. The absolute distance meter of claim 1, wherein the fiber switching
network further comprises:an optical circulator through which the emitted
light beam and the measure and reference light beams pass; anda partial
fiber retroreflector;wherein the optical circulator is optically
connected to the single channel detector, to the light source, and to the
at least one optical switch;wherein in the measure mode the emitted light
beam is sent from the light source through the optical circulator to the
at least one optical switch that is in the measure mode route and to the
target, and wherein the measure light beam from the target passes through
the at least one optical switch that is in the measure mode route and
through the optical circulator and to the single channel detector;
andwherein in the reference mode the emitted light beam is sent from the
light source through the optical circulator to the at least one optical
switch that is in the reference mode route and to the partial fiber
retroreflector, and wherein the reference light beam reflected from the
partial fiber retroreflector passes through the at least one optical
switch that is in the reference mode route and through the optical
circulator and to the single channel detector.

8. The absolute distance meter of claim 1, wherein the fiber switching
network further comprises:first and second fiber optic couplers through
which the emitted light beam and the measure and reference light beams
pass;wherein the first fiber optic coupler is optically connected to the
light source, to the at least one optical switch, and to the second fiber
optic coupler;wherein the second fiber optic coupler is optically
connected to the first fiber optic coupler, to the at least one optical
switch, and to the light source;wherein in the measure mode the emitted
light beam is sent from the light source through the first fiber optic
coupler, through the second fiber optic coupler and to the target, and
wherein the measure light beam from the target passes through the second
fiber optic coupler and to the least one optical switch that is in the
measure mode route and to the single channel detector; andwherein in the
reference mode the emitted light beam is sent from the light source
through the first fiber optic coupler and to the at least one optical
switch that is in the reference mode route, and to the single channel
detector as the reference light beam.

9. The absolute distance meter of claim 1, wherein the fiber switching
network further comprises:at least one fiber optic coupler through which
the emitted light beam and the measure and reference light beams
pass;second and third optical switches; anda partial fiber
retroreflector;wherein the at least one fiber optic coupler is optically
connected to the single channel detector, to the light source, and to the
at least one optical switch;wherein in the measure mode the emitted light
beam is sent from the light source through the at least one fiber optic
coupler to the at least one optical switch that is in the measure mode
route, to the second optical switch that is in the measure mode route,
and to the target, and wherein the measure light beam from the target
passes through the second optical switch that is in the measure mode,
through at least one optical switch that is in the measure mode route and
through the at least one fiber optic coupler and to the single channel
detector; andwherein in the reference mode the emitted light beam is sent
from the light source through the at least one fiber optic coupler to the
at least one optical switch that is in the reference mode route, to the
third optical switch that is in the reference mode route, and to the
partial fiber retroreflector, and wherein the reference light beam
reflected from the partial fiber retroreflector passes through the third
optical switch that is in the reference mode route, through at least one
optical switch that is in the reference mode route, and through the at
least one fiber optic coupler and to the single channel detector.

10. The absolute distance meter of claim 1, wherein the fiber switching
network further comprises:at least one fiber optic coupler through which
the emitted light beam and the measure and reference light beams pass;a
second optical switch; anda partial fiber retroreflector;wherein the at
least one fiber optic coupler is optically connected to the single
channel detector, to the light source, to the at least one optical
switch, and to the second optical switch;wherein in the measure mode the
emitted light beam is sent from the light source through the at least one
fiber optic coupler to the at least one optical switch that is in the
measure mode route and to the target, and wherein the measure light beam
from the target passes through the at least one optical switch that is in
the measure mode route and through the at least one fiber optic coupler
and to the single channel detector; andwherein in the reference mode the
emitted light beam is sent from the light source through the at least one
fiber optic coupler to the second optical switch that is in the reference
mode route and to the partial fiber retroreflector, and wherein the
reference light beam reflected from the partial fiber retroreflector
passes through the at least one optical switch that is in the reference
mode route and through the at least one fiber optic coupler and to the
single channel detector.

11. The absolute distance meter of claim 10, wherein the at least one
optical switch and the second optical switch each comprises an optical
modulator or attenuator that is driven between a first minimum level and
a second maximum level in which the at least one optical switch and the
second optical switch act as switches.

12. The absolute distance meter of claim 1, further comprising an optical
assembly that receives the emitted light beam from the fiber switching
network and transmits the emitted light beam towards the target, and that
receives the measure light beam from the target and transmits the measure
light beam toward the fiber switching network.

13. The absolute distance meter of claim 12, further comprising a visible
laser light source that emits a visible laser, wherein the optical
assembly includes optics that merges the emitted visible laser with the
emitted light beam from the fiber switching network to form a composite
emitted light beam and transmits the composite emitted light beam towards
the target.

14. The absolute distance meter of claim 1, wherein the target is a
retroreflector.

15. The absolute distance meter of claim 1, wherein the at least one
optical switch comprises a second optical fiber that terminates in a low
reflection termination.

16. The absolute distance meter of claim 1, wherein the light beam passes
through the at least one optical switch in a first direction and the
measure light beam passes through the at least one optical switch in a
second direction opposite to the first direction.

17. The absolute distance meter of claim 1, wherein a second optical
switch is cascaded with the at least one optical switch to increase
isolation between the two routes.

18. An absolute distance meter that determines a distance to a target,
comprising:a laser that emits a laser beam;a fiber switching network
having an optical switch that switches between two routes in response to
a switch control signal, a first route being a measure mode route in
which the laser beam is emitted from the fiber switching network through
an end of an optical fiber towards the target and is reflected back
through the end of the optical fiber as a measure light beam into the
fiber switching network, a second route being a reference mode route in
which the light beam comprises a reference light beam within the fiber
switching network;a single channel detector that detects the measure and
reference light beams in a multiplexed manner and provides an electrical
signal which corresponds to the detected measure and reference light
beams;electronics that controls timing of the switch control signal; anda
processor that processes the electrical signal to determine the distance
to the target.

19. The absolute distance meter of claim 18, wherein the processor
provides a modulation signal to the laser to modulate the laser.

20. The absolute distance meter of claim 18, wherein the processor
provides the switch control signal to control the switching of the
optical switch between the measure mode route and the reference mode
route.

21. The absolute distance meter of claim 18, wherein the absolute distance
meter is for use within a laser tracker, total station, laser scanner, or
handheld device, and wherein the target comprises a retroreflector.

22. The absolute distance meter of claim 18, further comprising an optical
assembly that receives the emitted light beam from the fiber switching
network and transmits the emitted light beam towards the target, and that
receives the measure light beam from the target and transmits the
reference light beam towards the fiber switching network.

[0002]The present invention relates to absolute distance meters, and more
particularly to an absolute distance meter having an optical fiber
switching network that reduces undesirable drift within the absolute
distance meter, thereby providing for more accurate distance
measurements.

BACKGROUND

[0003]Generally, an absolute distance meter (ADM) is a device that
determines the distance to a remote target. It does this by sending laser
light to the target and then collecting light that the target reflects or
scatters. An ADM may be used to measure distances in one dimension, as
might be seen, for example, in a consumer product available at a hardware
store. It may be attached into a more complex device having the ability
to measure quantities corresponding to additional dimensions (degrees of
freedom).

[0004]An example of a device of the latter type is the laser tracker,
which measures three-dimensional spatial coordinates. Exemplary systems
are described by U.S. Pat. No. 4,790,651 to Brown et al. and U.S. Pat.
No. 4,714,339 to Lau et al. The laser tracker sends a laser beam to a
retroreflector target held against a surface of interest or placed into a
fixed nest. The most common type of retroreflector target is the
spherically mounted retroreflector (SMR), which may comprise a
cube-corner retroreflector mounted within a sphere with the vertex of the
cube-corner at the sphere center.

[0005]A device that is closely related to the laser tracker is the laser
scanner. The laser scanner steps one or more laser beams to points on a
diffuse surface. The laser tracker and laser scanner are both
coordinate-measuring devices. It is common practice today to use the term
laser tracker to also refer to laser scanner devices having distance- and
angle-measuring capability. Another device closely related to the laser
tracker is the total station, typically used by surveyors. The broad
definition of laser tracker, which includes laser scanners and total
stations, is used throughout this document.

[0006]A radar device is similar to a laser tracker in that it emits and
receives electromagnetic waves and analyzes the received waves to learn
the distance to a target. Radars usually emit waves in the RF, microwave,
or millimeter region of the electromagnetic spectrum, whereas laser
trackers usually emit waves in the visible or near-infrared region.
Radars may be either bistatic or monostatic. Monostatic radars emit and
receive electromagnetic energy along a common path, whereas bistatic
radars emit and receive on different paths. Total stations may also be
either bistatic or mono static. Laser trackers used for high accuracy
industrial measurement, however, are monostatic.

[0007]To understand why laser trackers are monostatic, consider a beam
emitted by the laser tracker that travels to a retroreflector target and
is retroreflected back on itself. If a bistatic mode were used in the
tracker, the incident laser beam would strike off the retroreflector
center and the reflected laser beam would shift relative to the incident
beam. Small-size retroreflector targets of the sort often used with laser
trackers would not be compatible with such a bistatic device. For
example, a common type of retroreflector target is the 0.5-inch diameter
SMR. The cube-corner retroreflector in such an SMR typically has a clear
aperture diameter of about 0.3 inch, which equals about 7.5 mm. The
1/e2 irradiance diameter of a laser beam from a tracker might be
about this large or larger. Consequently, any shift in the laser beam
would cause the beam to be clipped by the SMR. This would result in an
unacceptably large drop in optical power returned to the tracker.

[0008]Bistatic geometry would also be problematic for a fiber-optic based
ADM system. In a monostatic laser tracker that launches laser light from
an optical fiber, a laser collimator can be made by placing the end face
of the optical fiber at the focal point of a collimating lens. On the
return path from the distant retroreflector, collimated laser light again
strikes the collimating lens, although in general the returning laser
beam may be off center with respect to the outgoing laser light. The
fiber end face is located at the focus of the collimating lens, which has
the effect of causing the light from the retroreflector target to be
efficiently coupled back into the fiber, regardless of where the beam
strikes the lens. In a bistatic device, alignment of the fiber-optic
receiving optics is much more challenging and coupling efficiency is much
lower.

[0009]One type of laser tracker contains only an interferometer (IFM)
without an absolute distance meter. If an object blocks the path of the
laser beam from one of these trackers, the IFM loses its distance
reference. The operator must then track the retroreflector to a known
location to reset to a reference distance before continuing the
measurement. A way around this limitation is to put an ADM in the
tracker. The ADM can measure distance in a point-and-shoot manner, as
described in more detail below. Some laser trackers contain only an ADM
without an interferometer. An exemplary laser tracker of this type is
described in U.S. Pat. No. 5,455,670 to Payne, et al. Other laser
trackers typically contain both an ADM and an interferometer. An
exemplary laser tracker of this type is described in U.S. Pat. No.
5,764,360 to Meier, et al.

[0010]A gimbal mechanism within the laser tracker may be used to direct a
laser beam from the tracker to the SMR. Part of the light retroreflected
by the SMR enters the laser tracker and passes onto a position detector.
A control system within the laser tracker can use the position of the
light on the position detector to adjust the rotation angles of the
mechanical azimuth and zenith axes of the laser tracker to keep the laser
beam centered on the SMR. In this way, the tracker is able to follow
(track) an SMR that is moved over the surface of an object of interest.

[0011]Angular encoders attached to the mechanical azimuth and zenith axes
of the tracker may measure the azimuth and zenith angles of the laser
beam (with respect to the tracker frame of reference). The one distance
measurement and two angle measurements performed by the laser tracker are
sufficient to completely specify the three-dimensional location of the
SMR.

[0012]One of the main applications for laser trackers is to scan the
surface features of objects to determine their geometrical
characteristics. For example, an operator can determine the angle between
two surfaces by scanning each of the surfaces and then fitting a
geometrical plane to each. As another example, an operator can determine
the center and radius of a sphere by scanning the sphere surface.

[0013]Prior to U.S. Pat. No. 7,352,446 to Bridges et al., an
interferometer, rather than an ADM, was required for the laser tracker to
scan moving targets. Until that time, absolute distance meters were too
slow to accurately find the position of a moving target. To get full
functionality with both scanning and point-and-shoot capability, early
laser trackers needed both an interferometer and an ADM.

[0014]A general comparison of interferometric distance measurement and
absolute distance measurement follows. In the laser tracker, an
interferometer (if present) may determine the distance from a starting
point to a finishing point by counting the number of increments of known
length (usually the half-wavelength of the laser light) that pass as a
retroreflector target is moved between the two points. If the beam is
broken during the measurement, the number of counts cannot be accurately
known, causing the distance information to be lost. By comparison, the
ADM in a laser tracker determines the absolute distance to a
retroreflector target without regard to beam breaks, which also allows
switching between a plurality of targets. Because of this, the ADM is
said to be capable of "point-and-shoot" measurement.

[0015]Although there are several sources of error in an interferometer
measurement, in most cases the dominant error is in the value of the
average wavelength of the laser light over its path through the air. The
wavelength at a point in space is equal to the vacuum wavelength of the
laser light divided by the index of refraction of the air at that point.
The vacuum wavelength of the laser is usually known to high accuracy
(better than one part in 10,000,000), but the average refractive index of
air is known less accurately. The refractive index of air is found by
first using sensors to measure the temperature, pressure, and humidity of
the air and then inserting these measured values into an appropriate
equation, such as the Ciddor equation or the Edlin equation.

[0016]However, the temperature, pressure, and humidity are not uniform
over space, and neither are the sensors perfectly accurate. For example,
an error in the average temperature of one degree Celsius causes an error
in the refractive index of about one part per million (ppm). As mentioned
above, the wavelength of light in air is inversely proportional to the
air refractive index.

[0017]Similarly, in an ADM, the so-called ADM wavelength of the amplitude
modulation envelope (also known as the ambiguity range) is inversely
proportional to the air group refractive index. Because of this
similarity, errors in measuring temperature, pressure, and humidity cause
errors in calculated distance that are approximately equal for ADM and
interferometer systems.

[0018]However, ADMs are prone to errors not found in interferometers. To
measure distance, an interferometer uses an electrical counter to keep
track of the number of times that two beams of light have gone in and out
of phase. The counter is a digital device that does not have to respond
to small analog differences. By comparison, ADMs are usually required to
measure analog values, such as phase shift or time delay, to high
precision.

[0019]In most high-performance ADMs, laser light is modulated, either by
applying an electrical signal to the laser source or by sending the laser
light through an external modulator such as an acousto-optic modulator or
electro-optic modulator. This modulated laser light is sent out of the
ADM to a remote target, which might be a retroreflector or a diffuse
surface. Light reflects or scatters off the remote target and passes, at
least in part, back into the ADM.

[0020]To understand the difficulties faced by ADMs, we consider two common
ADM architectures: temporally incoherent architecture and temporally
coherent architecture. In some temporally coherent systems, the returning
laser light is mixed with laser light from another location before being
sent to an optical detector that converts the light into an electrical
signal. This signal is decoded to find the distance from the ADM to the
remote target. In such systems, modulation may be applied to the
amplitude, phase, or wavelength of the laser light. In other temporally
coherent systems, several pure laser lines having different wavelengths
are combined before being sent to the retroreflector. These different
wavelengths of light are combined at the detector, thereby providing
"synthetic" modulation.

[0021]In temporally incoherent optical systems, light is not usually mixed
with light of another wavelength in an optical detector. The simplest
type of temporally incoherent system uses a single measure channel and no
reference channel. Usually laser light in such systems is modulated in
optical power. Light returning from the retroreflector strikes an optical
detector that converts the light into an electrical signal having the
same modulation frequency. This signal is processed electrically to find
the distance from the tracker to the target. The main shortcoming of this
type of system is that variations in the response of electrical and
optical components over time can cause jitter and drift in the computed
distance.

[0022]To reduce these errors in a temporally incoherent system, one
approach is to create a reference channel in addition to the measure
channel. This is done by creating two sets of electronics. One set of
electronics is in the measure channel. Modulated laser light returned
from the distant retroreflector is converted by an optical detector to an
electrical signal and passes through this set of electronics. The other
set of electronics is in the reference channel. The electrical modulation
signal is applied directly to this second set of electronics. By
subtracting the distance measured in the reference channel from the
distance found in the measure channel, jitter and drift are reduced in
ADM readings. This type of approach removes much of the variability
caused by electrical components, especially as a function of temperature.
However, it cannot remove variability arising from differences in
electro-optical components such as the laser and detector.

[0023]To reduce these errors further, part of the modulated laser light
can be split off and sent to an optical detector in the reference
channel. Most of the variations in the modulated laser light of the
measure and reference channels are common mode and cancel when the
reference distance is subtracted from the measure distance.

[0024]Despite these improvements, drift in such ADM systems can still be
relatively large, particularly over long time spans or over large
temperature changes. All of the architectures discussed above are subject
to drift and repeatability errors caused by variations in optical and
electrical elements that are not identical in the measure and reference
channels. Optical fibers used in ADM systems change optical path length
with temperature. Electrical assemblies used in ADM systems, such as
amplifiers and filters, change electrical phase with temperature.

[0025]A method and apparatus for greatly reducing the effects of drift in
an ADM within a laser tracker is taught in U.S. Pat. No. 6,847,436 to
Bridges, the contents of which are herein incorporated by reference. This
method involves use of a chopper assembly to alternately redirect
returning laser light to a measure or reference path. Although this
method works well, there is a limitation in the maximum rate of rotation
of the chopper wheel and hence in the data collection rate of the ADM.

[0026]A method of measuring the distance to a moving retroreflector is
taught in U.S. Pat. No. 7,352,446 to Bridges et al., the contents of
which are herein incorporated by reference. To obtain the highest
possible performance using the method of U.S. Pat. No. 7,352,446, the
distances are recomputed at a high rate, preferably at a rate of at least
10 kHz. It is difficult to make a mechanical chopper as in U.S. Pat. No.
6,847,436 with a data rate this high. Hence another method needs to be
found to solve the ADM drift problem.

[0027]It is possible to correct for drift in a distance meter by
mechanically switching an optics beam between two free-space optical
paths. One optical path, which is called the reference path, is internal
to the instrument. The second optical path, which is called the measure
path, travels out from the instrument to the object being measured and
then back to the instrument. Light from the measure and reference paths
strikes a single optical detector. Because of the action of the
mechanical switch, the light from the two reference paths does not strike
the single optical detector at the same time. The mechanical switch may
be a mechanically actuated optical component such as a mirror, prism,
beam splitter, or chopper wheel. The actuator may be a solenoid, motor,
voice coil, manual adjuster, or similar device. Because the optical
detector and electrical circuitry is the same for the measure and
reference paths, almost all drift error is common mode and cancels out.
Examples of inventions based on this method include U.S. Pat. No.
3,619,058 to Hewlett et al.; U.S. Pat. No. 3,728,025 to Madigan et al.;
U.S. Pat. No. 3,740,141 to DeWitt; U.S. Pat. No. 3,779,645 to Nakazawa et
al.; U.S. Pat. No. 3,813,165 to Hines et al.; U.S. Pat. No. 3,832,056 to
Shipp et al.; U.S. Pat. No. 3,900,260 to Wendt; U.S. Pat. No. 3,914,052
to Wiklund; U.S. Pat. No. 4,113,381 to Epstein; U.S. Pat. No. 4,297,030
to Chaborski; U.S. Pat. No. 4,453,825 to Buck et al.; U.S. Pat. No.
5,002,388 to Ohishi et al.; U.S. Pat. No. 5,455,670 to Payne et al.; U.S.
Pat. No. 5,737,068 to Kaneko et al.; U.S. Pat. No. 5,880,822 to Kubo;
U.S. Pat. No. 5,886,777 to Hirunuma; U.S. Pat. No. 5,991,011 to Damm;
U.S. Pat. No. 6,765,653 to Shirai et al.; U.S. Pat. No. 6,847,436 to
Bridges; U.S. Pat. No. 7,095,490 to Ohtomo et al.; U.S. Pat. No.
7,196,776 to Ohtomo et al.; U.S. Pat. No. 7,224,444 to Stierle et al.;
U.S. Pat. No. 7,262,863 to Schmidt et al.; U.S. Pat. No. 7,336,346 to
Aoki et al.; U.S. Pat. No. 7,339,655 to Nakamura et al.; U.S. Pat. No.
7,471,377 to Liu et al.; U.S. Pat. No. 7,474,388 to Ohtomo et al.; U.S.
Pat. No. 7,492,444 to Osada; U.S. Pat. No. 7,518,709 to Oishi et al.;
U.S. Pat. No. 7,738,083 to Luo et al.; and U.S. Published Patent
Application No. US2009/0009747 to Wolf et al. Because all of these
patents use mechanical switches, which are slow, none can switch quickly
enough to be used in an ADM that accurately measures a moving
retroreflector.

[0028]Another possibility is to correct drift only in the electrical, and
not the optical, portion of a distance meter. In this case, light from
the reference optical path is sent to the reference optical detector and
light from the measure optical path is sent to the measure optical
detector. The electrical signals from the reference and optical detectors
travel to an electrical switch, which alternately routes the electrical
signals from the two detectors to a single electrical unit. The
electrical unit processes the signals to find the distance to the target.
Examples of inventions based on this method include: U.S. Pat. No.
3,365,717 to Holscher; U.S. Pat. No. 5,742,379 to Reifer; U.S. Pat. No.
6,369,880 to Steinlechner; U.S. Pat. No. 6,463,393 to Giger; U.S. Pat.
No. 6,727,985 to Giger; U.S. Pat. No. 6,859,744 to Giger; and U.S. Pat.
No. 6,864,966 to Giger. Although the use of an electrical switch can
reduce drift in the electrical portion of an ADM system, it cannot remove
drift from the optical portion, which is usually as large or larger than
the drift in the electrical portion. In addition, it is difficult to
implement an electrical switching system that can switch quickly enough
to avoid a phase shift in electrical signals modulated at several GHz.
Because of their limited utility and difficulty of implementation,
electrical switches are not a good solution for correcting drift in an
ADM.

[0029]For a bistatic distance meter, there are two references that discuss
the use of fiber optic switches. U.S. Published Patent Application No.
US2009/0046271 to Constantikes teaches a method in which one fiber switch
is placed in the outgoing beam path and a second fiber switch is placed
in the returning beam path. These two fiber optic switches are switched
at the same time to either permit light from the measure or reference
path to reach the optical detector. U.S. Pat. No. 4,689,489 to Cole
teaches use of a fiber switch in which light from the return port of the
bistatic distance meter is into one port of a switch and light from the
outgoing beam is fed into the second port of the switch. The fiber-switch
architectures described in these references apply only to bistatic
devices and cannot be used with laser trackers for reasons discussed
earlier.

[0030]There is a need for an ADM that accurately measures moving targets
with little drift. It must be monostatic and minimize drift in both
optical and electrical components.

SUMMARY

[0031]According to an aspect of the present invention, an absolute
distance meter (ADM) that determines a distance to a target includes a
light source that emits an emitted light beam. The ADM also includes a
fiber switching network having at least one optical switch that switches
between at least two positions in response to a switch control signal, a
first one of the positions enabling a measure mode in which the emitted
light beam is emitted from the fiber switching network towards the target
and is reflected back as a measure light beam into the fiber switching
network, a second one of the positions enabling a reference mode in which
the light beam comprises a reference light beam within the fiber
switching network. The ADM further includes a single channel detector
that detects the measure and reference light beams in a temporally spaced
multiplexed manner and provides an electrical signal which corresponds to
the detected measure and reference light beams. Also, the ADM includes a
single channel signal processor that processes the electrical signal and
provides a conditioned electrical signal in response thereto, and a data
processor that processes the conditioned electrical signal to determine
the distance to the target.

BRIEF DESCRIPTION OF THE DRAWINGS

[0032]Embodiments will now be described, by way of example only, with
reference to the accompanying drawings which are meant to be exemplary,
not limiting, and wherein like elements are numbered alike in several
Figures, in which:

[0033]FIG. 1 is a perspective view of an exemplary laser tracker sending a
laser beam to an external retroreflector; and

[0034]FIG. 2A is a block diagram of a tracker electro-optics assembly
including an ADM with an optical fiber switching network, visible laser,
and tracker optics; and

[0035]FIG. 2B is a block diagram of a tracker electro-optics assembly
including an ADM with an optical fiber switching network, incremental
distance meter assembly, and tracker optics; and

[0036]FIG. 3 is a block diagram of a tracker electro-optics assembly
including an ADM with an optical fiber switching network and tracker
optics; and

[0037]FIG. 4 is a block diagram of a tracker electro-optics assembly
including an ADM with an optical fiber switching network and simplified
optics; and

[0038]FIG. 5 shows an optical fiber switching network that includes a
fiber optic switch, optical coupler, and a partial fiber retroreflector
according to an embodiment of the present invention; and

[0039]FIG. 6 shows an optical fiber switching network that includes a
fiber optic switch, optical circulator, and partial fiber retroreflector
according to another embodiment of the present invention; and

[0040]FIG. 7 shows an optical fiber switching network that includes two
fiber optic couplers and a fiber-optic switch according to yet another
embodiment of the present invention; and

[0041]FIG. 8 shows an optical fiber switching network in which multiple
fiber optic switches are combined to increase optical isolation according
to still another embodiment of the present invention; and

[0042]FIG. 9 shows an optical fiber switching network in which the
switching action is performed by optical modulators or optical
attenuators according to another embodiment of the present invention; and

[0043]FIG. 10 is a block diagram of exemplary ADM electronics used in
embodiments of the present invention; and

[0044]FIG. 11 is a block diagram of the data processor used in embodiments
of the present invention; and

[0045]FIG. 12 is a graph of an exemplary signal from an ADM system; and

[0046]FIG. 13 is a graph of an exemplary switching signal;

[0047]FIG. 14 is a graph of an exemplary gating signal;

[0048]FIG. 15 is a block diagram of a processing system used in
embodiments of the present invention; and

[0049]FIG. 16 is a block diagram of ADM electronics used in embodiments of
the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0050]An exemplary laser tracker 10 is illustrated in FIG. 1. An exemplary
gimbaled beam-steering mechanism 12 of laser tracker 10 comprises zenith
carriage 14 mounted on azimuth base 16 and rotated about azimuth axis 20.
Payload 15 is mounted on zenith carriage 14 and rotated about zenith axis
18. Zenith mechanical rotation axis 18 and azimuth mechanical rotation
axis 20 intersect orthogonally, internally to tracker 10, at gimbal point
22, which is typically the origin for distance measurements. Laser beam
46 virtually passes through gimbal point 22 and is pointed orthogonal to
zenith axis 18. In other words, laser beam 46 is in the plane normal to
zenith axis 18. Laser beam 46 is pointed in the desired direction by
rotation of payload 15 about zenith axis 18 and by rotation of zenith
carriage 14 about azimuth axis 20. Zenith and azimuth angular encoders,
internal to the tracker (not shown), are attached to zenith mechanical
axis 18 and azimuth mechanical axis 20 and indicate, to high accuracy,
the angles of rotation. Laser beam 46 travels to external retroreflector
26 such as the spherically mounted retroreflector (SMR) described above.
By measuring the radial distance between gimbal point 22 and
retroreflector 26 and the rotation angles about the zenith and azimuth
axes 18, 20, the position of retroreflector 26 is found within the
spherical coordinate system of the tracker.

[0051]Laser beam 46 may comprise one or more laser wavelengths, as will be
described in the discussion that follows. For the sake of clarity and
simplicity, a steering mechanism of the sort shown in FIG. 1 is assumed
in the following discussion. However, other types of steering mechanisms
are possible. For example, it would be possible to reflect a laser beam
off a minor rotated about the azimuth and zenith axes. The techniques
described here are applicable, regardless of the type of steering
mechanism.

[0053]There are many ways to modulate light. One type of modulation is of
optical power, with the modulation signal usually either sinusoidal or
pulsed. Another type of modulation is of optical wavelength. This type of
modulation is sometimes used in coherent laser distance meters.
Modulation may be applied directly to the light source or to an external
modulator, such as an electro-optic modulator, to vary the power,
polarization, or phase of the laser light. The method described in this
disclosure is applicable to any of these types of modulation. Light can
come from a laser, superluminescent diode, or any other type of optical
emitter. In the text below, the light source is often referred to as a
laser, but this should not be taken to limit the type of light source
that could be used.

[0059]Position detector 130 produces an electrical signal that indicates
the position of the spot of light on position detector 130. Position
detector 130 may be any type of detector that indicates the position of
the returning light beam. For example, it may be a position sensitive
detector such as a lateral effect detector or quadrant detector or it may
be a photosensitive array such as CCD or CMOS array. The retrace point of
the position detector is defined as the point that laser beam 126 strikes
if laser beam 46 strikes the center of retroreflector 26. When laser beam
46 moves off the center of retroreflector 26, laser beam 126 moves off
the retrace point and causes the position detector 130 to generate an
electrical error signal. A servo system (not shown) processes this error
signal to activate motors (not shown) that turn laser beam 46 from laser
tracker 10 toward the center of the external retroreflector 26. By this
means, the laser beam from tracker 10 is made to track the movement of
retroreflector 26.

[0060]Dichroic beam splitter 114 transmits the returning ADM laser light
through ADM beam collimator 140, where it is coupled into optical fiber
501. The laser light travels back into fiber switching network 200, and a
part of it travels through optical fiber 230 to ADM electronics 300. ADM
electronics 300 converts the optical signal into an electrical signal and
conditions the electrical signal in a way appropriate for the particular
type of modulation applied to the laser light. The signal from ADM
electronics 300 is sent to data processor 400, which processes the signal
to find result 420, the distance from tracker gimbal point 22 to
retroreflector target 26.

[0061]The components of tracker electro-optics assembly 250A, 250B may be
located entirely within tracker payload 15, located partly within tracker
payload 15 and partly within azimuth base 16, or located entirely within
azimuth base 16. If ADM or interferometer components are located in
azimuth base 16, these may be connected to optical components by routing
fiber optic cables through the mechanical azimuth and zenith axes into
payload 15. This method is shown in WO 2003/062744, which is incorporated
herein by reference. Alternatively, if ADM or interferometer components
are located in azimuth base 16, the light emitted by ADM laser 102 or
stable laser 182 may be sent through free space to a beam steering mirror
located in the payload. This method is shown in U.S. Pat. No. 4,714,339
to Lau et al.

[0062]Optical fiber switching network 200 provides a means of routing and
switching optical signals to and from optical assembly 190. Fiber
switching network 200 is described in more detail below.

[0063]It is possible to eliminate visible-light laser 110 in FIG. 2A or
incremental distance meter assembly 180 in FIG. 2B. In this case,
visible-beam launch 150 is not necessary. The resulting electro-optics
assembly 350 is shown in FIG. 3. This architecture might be appropriate
if an IFM were not needed and if ADM laser 102 emitted visible laser
light. It might also be appropriate if the IFM were not needed and if a
visible pointer beam was not needed.

[0064]For handheld distance meters or other instruments that do not track,
the architecture can be further simplified by eliminating tracking
assembly 170 and possibly beam expander 160. The resulting ADM distance
meter 450 is shown in FIG. 4.

[0065]FIGS. 2A, 2B, 3, and 4 all contain ADM assembly 2000, which contains
optical fiber switching network 200. The benefit of fiber switching
network 200 is that it enables a reduction in the drift of the ADM
distance readings. The reason for this reduction can be understood by
considering ADM electronics 300 in more detail. A specific embodiment for
the ADM electronics is considered in the discussion that accompanies
FIGS. 10 and 11; that is, in conjunction with a laser tracker. However,
the advantages of the fiber switching network for reducing drift in an
ADM system applies more generally to ADM systems and may include for
example pulsed time-of-flight ADMs, chirped ADMs, and coherent as well as
incoherent ADMs. To explain how fiber switching network 200 enables the
reduction in drift, reference is now made to FIG. 16, which describes the
elements of ADM electronics 300 in more general terms.

[0066]In FIG. 16, ADM electronics 300 comprises laser transmitter 310,
single channel laser receiver 320, single channel signal line 332, and
interconnection lines 330, 334, and 336. Laser transmitter 310 may
generate a variety of signals. A signal from interconnection line 330 is
used to modulate ADM laser 102. In addition, most types of ADM systems
generate one or more additional signals used in processing of the signal
in single channel receiver 320. The combination of such signals is
referred to here as the single channel signal 332, for reasons that will
become clear in the discussion that follows.

[0067]Single channel receiver 320 comprises single detector 322 and single
channel electronics 324. Light arrives at single detector 322 over
interconnection line 336, which is a fiber optic cable attached to fiber
switching network 200. Single detector 322 converts the optical signal
from 336 to an electrical signal. This electrical signal is processed by
single channel electronics, and the resulting processed signal is sent
over interconnection line 334 to data processor 400.

[0068]The drift seen in ADM systems is generally the result of changes in
the electrical and optical systems over time and especially with respect
to changes in temperature. In the Background section of this document, it
was explained that ADM systems often try to remove the effects of such
changes by subtracting the readings of a reference channel from those of
a measure channel. As explained, the signal in the reference channel can
be optical or electrical, with an optical reference signal generally
producing the highest performance. The use of two channels in this way
can only correct drift to a limited degree because two separate
electrical channels are required in the receiver unit--one for the
measure channel and one for the reference channel. If the reference
signal is optical, the receiver unit must also provide two separate
optical detectors--one for the measure channel and one for the reference
channel. However, the electrical and optical components within the two
channels are not identical and neither are the temperatures of the
components in each of the channels. Consequently, drift seen within the
measure and reference channels is not completely common mode and does not
completely cancel out.

[0069]By using a fiber switching network to multiplex optical signals, it
is possible to use a single detector to serve both measure and reference
channels. It is also possible to use a single electrical channel, rather
than two electrical channels, in the receiver. Because there is only a
single electrical receiver channel, any electrical signals supplied by
transmitter 310 need to be provided in only a single channel. The result
of the single optical detector, the single electrical receiver channel,
and the single channel signals from the transmitter is a nearly complete
cancellation of drift effects. The resulting ADM system is nearly drift
free.

[0071]Electrical connection 470 sends to fiber-optic switch 500 an
electrical signal that controls whether the optical signal is routed to
optical fiber 501 or optical fiber 502. If switch 500 routes light to
optical fiber 501, light passes from stable ferrule 142 through the
tracker and out to retroreflector 26. The returning laser light travels
to fiber-optic switch 500, through coupler 206, through fiber 230, and
into ADM electronics 300. Light that travels along this path to and from
the retroreflector is said to be in the measure path and, during this
time, the tracker is said to be in the measure mode.

[0072]If switch 500 routes light to optical fiber 502, light passes to
partial fiber retroreflector 505, which reflects a fraction of laser
light back through coupler 206, through fiber 230, and into ADM
electronics 300. Light that travels internal to the tracker by reflecting
off partial fiber retroreflector 505 is said to be in the reference path
and, during this time, the tracker is said to be in the reference mode.

[0073]Fiber coupler 206 is preferably a 50/50 coupler, also known as a 3
dB coupler. For light injected into a 50/50 coupler 206 by ADM laser 102,
50% of the laser light goes to optical fiber 510 and 50% goes to optical
fiber 503. For light injected into coupler 206 from the reverse
direction, 50% of the returning light goes to ADM laser 102 and 50% of
the returning light goes to ADM electronics 300. Faraday isolation is
provided within ADM laser 102 to prevent light that passes through fiber
coupler 206 to ADM laser 102 from destabilizing the laser.

[0074]The amount of light returned to optical fiber 501 after the light
has traveled to retroreflector 26 depends on a number of factors
including the distance to the retroreflector, the diameter and tilt of
the retroreflector, and the coupling efficiency of the ADM beam
collimator 140. The reflectance of partial fiber retroreflector 505 is
preferably selected to reflect laser power approximately equal to the
average of powers returned by retroreflector 26 under different
measurement conditions.

[0075]Fiber-optic switch 500 should preferably have optical isolation
between the two switching positions of at least 20 dB. This means that,
when the switch is in the up position, the amount of optical power that
leaks into the down position is less than that applied to the up position
by a factor of at least 100. After reflecting and retracing the path,
isolation is reduced by another factor of 100, so that the overall
effective isolation is a factor of 104, or 40 dB. Switches with
lower levels of isolation can be used by combining them to increase their
overall isolation, as explained below.

[0076]In addition to optical isolation, fiber-optic switch 500 should
preferably have optical return loss of at least 40 dB. This means that
the light reflected back by the switch should be reduced by a factor of
at least 10,000 compared to the incident light. This ensures that
excessive unwanted light is not reflected onto the light traveling on the
desired path and thereby reducing the accuracy of the measurement.

[0078]The advantage of a three-port circulator, such as 610 in FIG. 6,
compared to a four-port fiber optic coupler, such as 206 in FIG. 5, is
that no power is lost to the fourth port, which in 206 of FIG. 5 is
dissipated in low-reflection termination 208. The disadvantage of a
circulator is that it will generally have some level of polarization mode
dispersion (PMD). As a result, any change in polarization state of light
returned on optical fiber 501 or 502 can result in a delay in the phase
of the modulated light, thereby producing an error in the reported ADM
distance.

[0080]Light returned to stable ferrule 142 travels back through optical
fiber 501 to second optical coupler 206. Part of the return light from
second optical coupler 206 travels to optical switch 700. Another part of
the return light from second optical coupler 206 travels back through
optical fiber 716 to first optical coupler 204. Part of this return light
goes through optical fiber 104 to ADM laser 102, where it is blocked by a
Faraday isolator built into the laser. Another part of the return light
travels through optical fiber 715 to low reflection termination 210.

[0083]In the measure mode, switch 500 connects optical fiber 503 to
optical fiber 812, and switch 810 connects optical fiber 812 to optical
fiber 501. Also, in the measure mode, switch 820 connects optical fiber
502 to optical fiber 822 that leads to low-reflection termination 826.
Suppose that the isolation of each switch 500, 810, 820 is 20 dB. This
means, for example, that less than 0.01 of the optical power will pass
through to the undesired path in a particular switch. In this case, less
than 0.01 of the optical power present on optical fiber 503 will pass to
optical fiber 502, and less than 0.0001 will pass to fiber 824. This
light reflected by partial fiber retroreflector 505 will be further
reduced by a factor of 0.0001 in passing back to optical fiber 503. In
other words, the reflected optical power is decreased by a factor of at
least 10-8=-80 dB compared to the outgoing optical power on optical
fiber 503.

[0084]In the reference mode, switch 500 connects optical fiber 503 to
optical fiber 502, and switch 820 connects optical fiber 502 to optical
fiber 824 that leads to partial fiber retroreflector 505. Also, in the
reference mode, switch 810 connects optical fiber 812 to optical fiber
814 that leads to low-reflection termination 816. As in the previous
case, for switches each having 20 dB of isolation, the resulting power
returned to optical fiber 503 is reduced to less than 10-8=80 dB
times the original amount.

[0086]A specific embodiment of ADM electronics 300 is now considered. This
particular embodiment will be referred to as ADM electronics 3000 as is
shown in FIG. 10. ADM electronics 3000 converts the light output of fiber
switching network 200 in either the measure mode or reference mode into a
digital electrical signal for processing by the data processor 400 and
also generates modulation signal for ADM laser 102. The input to ADM
electronics 3000 is fiber optic 230 and the outputs are electrical
modulation signal 360 and conditioned electrical signal 460. U.S. Pat.
No. 7,352,446 to Bridges et al., which is incorporated by reference,
discloses details for similar ADM electronics 3000.

[0087]ADM electronics 3000 of FIG. 10 comprises frequency reference 3002,
synthesizer 3004, detector 3006, mixers 3010, amplifiers 3014, 3018,
frequency divider 3024, and analog-to-digital converter (ADC) 3022.
Frequency reference 3002 provides the time base for the ADM and should
have low phase noise and low frequency drift. The frequency reference may
be an oven-controlled crystal oscillator (OCXO), rubidium oscillator, or
any other highly stable frequency reference. Preferably the oscillator
frequency should be accurate and stable to within a small fraction of a
part per million. The signal from the frequency reference is put into the
synthesizer, which generates three signals. The first signal is at
frequency fRF and modulates the optical power of ADM laser 102. This
type of modulation is called intensity modulation (IM). Alternatively, it
is possible for the first signal at frequency fRF to modulate the
electric field amplitude, rather than the optical power, of the laser
light from ADM laser 102. This type of modulation is called amplitude
modulation (AM). The second and third signals, both at the frequency
fLO, go to the local-oscillator ports of mixer 3010.

[0088]Fiber-optic cable 230 carries laser light. The light in this
fiber-optic cable 230 is converted into electrical signals by detector
3006. This optical detector 3006 sends the modulation frequency fRF
to amplifier 3014 and then to mixers 3010. Mixer 3010 produces two
frequencies, one at |fLO-fRF| and one at |fLO+fRF|.
These signals travel to low-frequency amplifier 3018. Amplifier 3018
blocks the high-frequency signals so that only the signals at the
intermediate frequency (IF), fIF=|fLO-fRF| pass through to
the analog-to-digital converter (ADC) 3022. The frequency reference 3002
sends a signal into frequency divider 3024, which divides the frequency
of the reference 3002 by an integer N to produce a sampling clock. In
general, the ADC may decimate the sampled signals by an integer factor M,
so that the effective sampling rate is fREF/NM. This effective
sampling rate should be an integer multiple of the intermediate frequency
fIF.

[0089]The timing electronics 472 may comprise a frequency divider chip and
a microprocessor or field-programmable gate array. The frequency divider
chip divides the frequency of the signal from frequency reference 3002 to
a lower frequency. This frequency is applied to the microprocessor or
field-programmable gate array that uses its internal processing
capability to provide the required timing signals shown in FIGS. 13 and
14.

[0090]Here are frequencies for an exemplary ADM: The frequency reference
is fREF=20 MHz. The synthesizer RF frequency that drives the laser
is fRF=2800 MHz. The synthesizer LO frequency that is applied to the
mixers is fLO=2800.01 MHz. The difference between the LO and RF
frequencies is the intermediate frequency of fIF=10 kHz. The
frequency reference is divided by N=10, to produce a 2-MHz frequency that
is applied to the ADC as a sampling clock. The ADC has a decimation
factor of M=8, which produces an effective sampling rate of 250 kHz.
Since the IF is 10 kHz, the ADC takes 25 samples per cycle.

[0091]The ADC sends the sampled data to data processor 400 for analysis.
Data processors include digital signal processor (DSP) chips and
general-purpose microprocessor chips. The processing performed by these
processors is described below.

[0092]As shown in FIGS. 2-4, ADM electronics 3000 generates a signal that
travels over electrical connection 470 to switch fiber switching network
200 between measure and reference modes. In addition, data processor 400
converts the digital output of ADM electronics 3000 to result 420, which
is a numerical distance value. One exemplary embodiment of data processor
400 is data processor 400A shown in FIG. 11. The input to data processor
400A is electrical interface 460 to ADM electronics 3000 and the output
is result 420. U.S. Pat. No. 7,352,446, incorporated by reference above,
also discloses details for a similar data processor 400.

[0093]Data processor 400 of FIG. 11 takes the digitized data from ADC 3022
and derives from it the distance from the tracker to external
retroreflector 26. FIG. 11 refers to this distance as the RESULT 420.
Data processor 400 comprises digital signal processor 410, microprocessor
450, and crystal oscillators 402, 404.

[0094]Analog-to-digital converter 3022 sends sampled data to DSP 410. This
data is routed to a program that runs within the DSP. This program
contains three main functions: phase-extractor function 420, compensator
function 422, and Kalman-filter function 424. The purpose of the
phase-extractor function is to determine the phases of the signals, that
is, the phases of the signals that pass through the detector 3006. To
determine these phases, the modulation range must first be calculated.
Modulation range is defined as the round-trip distance traveled by the
ADM laser light in air for the phase of the laser modulation to change by
2 pi radians.

[0095]To synchronize the ADM measurement to the measurements of the
angular encoders and position detector, counter 414 determines the
difference in time between the sync pulse and the last state distance. It
does this in the following way. Crystal oscillator 404 sends a
low-frequency sine wave to frequency divider 452, located within
microprocessor 450. This clock frequency is divided down to fSYNC,
the frequency of the sync pulse. The sync pulse is sent over a device bus
to DSP, angular encoder electronics, and position-detector electronics.
In an exemplary system, the oscillator sends a 32.768 kHz signal through
frequency divider 452, which divides by 32 to produce a sync-pulse
frequency fSYNC=1.024 kHz. The sync pulse is sent to counter 414,
which resides within DSP 410. The counter is clocked by crystal 402,
which drives a phase-locked loop (PLL) device 412 within the DSP. In the
exemplary system, oscillator 402 has a frequency of 30 MHz and PLL 412
doubles this to produce a clock signal of 60 MHz to counter 414. The
counter 414 determines the arrival of the sync pulse to a resolution of
1/60 MHz=16.7 nanoseconds. The phase-extractor function 420 sends a
signal to the counter when the ADC 322 has sent all the samples for one
cycle. This resets counter 414 and begins a new count. The sync pulse
stops the counting of counter 412. The total number of counts is divided
by the frequency to determine the elapsed time. Since the time interval
in the above equations was set to one, the normalized time interval
tNORM is the elapsed time divided by the time interval. The state
distance xEXT extrapolated to the sync pulse event is

xEXT=xk+vk tNORM.

The Kalman-filter function 424 provides the result, which is the distance
from the tracker to external retroreflector 26.

[0096]It is important to recognize that the method of using fiber-optic
switches described herein is not limited to a phase-based distance
measurement method, of which the exemplary embodiment of FIG. 10 is one
example. For example, fiber optic switches can equally well be used with
a pulsed time-of-flight distance meter.

[0097]FIG. 12 shows an example of the multiplexed 1300 signal that emerges
from signal conditioner 3018 of FIG. 10 and enters analog-to-digital
converter (ADC) 3022 of the same figure. This type of multiplexed signal
might be produced by a phase-based ADM. In FIG. 12, the larger amplitude
represents the signal from the measure channel, and the smaller amplitude
represents the signal from the reference channel. The reference and
measure signals are multiplexed together by fiber switching network 200.
In the example shown in FIG. 12, the frequency of the sinusoidal is 100
kHz, and the corresponding period is 0.01 milliseconds=10 microseconds.
Numerical result 420 has, in this example, an output frequency of 10 kHz
and a corresponding period of 0.1 milliseconds=100 microseconds.

[0098]In general, the act of switching between measure and reference
signals causes some transients to appear in the output signals of
electrical and opto-electric components of ADM electronics 3000. If these
transient signals, which are read by ADC 3022, were included in the
calculations of data processor 400, an erroneous result 420 would occur.
To avoid this problem, it is important that transients have died out in
the raw data processed by data processor 400 to get result 420.

[0099]In the example considered here, only 80 microseconds of each 100
microsecond period are processed, and the other 20 microseconds are
discarded. Of the 80 microseconds that are retained, 20 microseconds (2
sinusoidal periods) are retained from the reference channel and 60
microseconds (6 sinusoidal periods) are retained from the measure
channel.

[0100]FIG. 13 shows timing signal 1200 from electrical connection 470.
Measure mode begins when timing signal 1200 goes to high value 1210, and
reference mode begins when timing signal 1200 goes to low value 1230.
FIG. 14 shows the gating signal 1250 that indicates when data 460 is
considered valid. A high gating signal 1260 indicates that the reference
signal is valid. A high gating signal 1265 indicates that the measure
signal is valid. A low gating signal 1255 indicates that no signal is
valid.

[0101]The methods of algorithms discussed above are implemented by means
of processing system 1500 shown in FIG. 15. Processing system 1500
comprises tracker processing unit 1510 and optionally computer 80.
Processing unit 1510 includes at least one processor, which may be a
microprocessor, digital signal processor (DSP), field programmable gate
array (FPGA), or similar device. Processing capability is provided to
process information and issue commands to internal tracker processors.
Such processors may include position detector processor 1512, azimuth
encoder processor 1514, zenith encoder processor 1516, indicator lights
processor 1518, ADM processor 400, interferometer (IFM) processor 1522,
and camera processor 1524. Auxiliary unit processor 1570 optionally
provides timing and microprocessor support for other processors within
tracker processor unit 1510. Preferably, it communicates with other
processors by means of device bus 1530, which preferably transfers
information throughout the tracker by means of data packets, as is well
known in the art. Preferably, computing capability is distributed
throughout tracker processing unit 1510, with DSPs and FPGAs performing
intermediate calculations on data collected by tracker sensors. The
results of these intermediate calculations are returned to auxiliary unit
processor 1570. Auxiliary unit 1570 may be attached to the main body of
laser tracker 10 through a long cable, or it may be pulled within the
main body of the laser tracker so that the tracker attaches directly (and
optionally) to computer 80. Preferably, auxiliary unit 1570 is connected
to computer 80 by connection 1540, which is preferably an Ethernet cable
or wireless connection. Auxiliary unit 1570 and computer 80 may be
connected to the network through connections 1542, 1544, which are
preferably Ethernet cables or wireless connections.

[0102]While the description above refers to particular embodiments of the
present invention, it will be understood that many modifications may be
made without departing from the spirit thereof. The accompanying claims
are intended to cover such modifications as would fall within the true
scope and spirit of the present invention.

[0103]The presently disclosed embodiments are therefore to be considered
in all respects as illustrative and not restrictive, the scope of the
invention being indicated by the appended claims, rather than the
foregoing description, and all changes which come within the meaning and
range of equivalency of the claims are therefore intended to be embraced
therein.